It is well known that image formation with silver halide photography requires two processes, i.e., a process of exposure for producing a latent image and a process of development for converting the produced latent image into a silver image or dye image (see, for example, T. H. James, The Theory of the Photographic Process).
Latent image formation by imagewise exposure chemically causes an extremely slight change in the silver halide crystals and, therefore, the latent image itself is essentially unstable. Accordingly, the latent image is liable to be attenuated with the lapse of time after exposure till development or is susceptible to intensification.
In the field of photography, this attenuation of the latent image is called latent image fading, and the latter phenomenon is called latensification.
The behavior of a latent image is generally dependent upon the conditions under which exposed light-sensitive materials are stored. For example, considerable latent image fading or latensification results under storage at elevated temperatures, whereas less latent image fading or latensification results under storage at low temperatures.
The simplest technique for avoiding disadvantages of latent image fading or latensification is to conduct development processing immediately after image-wise exposure, and the second simplest technique is to store exposed light-sensitive materials at low temperatures under cooling during the period between exposure and development processing.
These techniques are simplest and easiest from the chemical point of view, but they are not always convenient for the photographer. In actual conditions or manners under or in which light-sensitive materials are used, they are in some cases several months between exposure and development processing. With light-sensitive materials, particularly multilayered color photographic light-sensitive materials, in which the latent image fading or latensification differs in different layers, there results poor color balance, leading to deterioration of color reproduction.
In recent years, hardeners have come into use which have active vinyl groups and which exhibit a rapid hardening effect and cause less change in the hardening effect after a long period of time, called "post hardening". Examples of such compounds are described in U.S. Pat. Nos. 3,689,274, 3,868,257, 4,028,320, 4,088,495, 4,137,082 and 4,173,481, British Pat. No. 1,397,905, Japanese Patent Application (OPI) Nos. 66960/78 and 30022/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and Japanese Patent Publication No. 46495/77.
However, the use of such active vinyl group-containing hardener concurrently may cause latensification.
Techniques for preventing latensification include those in U.S. Pat. No. 3,881,933 which describes the use of hardeners which release an acid in an emulsion film in order to reduce the pH, Japanese Patent Application (OPI) No. 6725/73 which describes adding a rhodium compound and an iridium compound upon emulsification of silver halide, Japanese Patent Publication No. 23250/82 which describes adding a mercapto hetero ring compound to a silver halide emulsion prepared by using a rhodium compound, and Japanese Patent Application (OPI) No. 127714/78 which describes adding a hydroxyazaindolidine compound and a nitrogen-containing hetero ring compound having a mercapto group to a silver halide emulsion prepared by using a rhodium compound. However, these techniques require specially prepared silver halide emulsions or require silver halide emulsions spectrally sensitized with particular sensitizing dyes, or provide only insufficient effects, thus none of these techniques being fully satisfactory.